Catalytic humidifier and heater for the fuel stream of a fuel cell

ABSTRACT

A method and apparatus are provided for humidifying fuel, and optionally oxidant, supplied to a fuel cell system, which can be a single fuel cell or a multiplicity of fuel cells. A catalytic reactor is provided, which is supplied with a portion of the fuel and the oxidant. The fuel is supplied in excess of the oxidant to the catalytic reactor, so as to generate a stream of fuel which is both heated and humidified. For a closed system, a heated and humidified fuel flow, and optionally a heated and humidified oxidant flow, are mixed with additional flows of these gases supplied to the fuel cell.

[0001] The present application is a divisional of U.S. Ser. No.09/592,950 that was filed on Jun. 13, 2000 (now pending).

FIELD OF THE INVENTION

[0002] This invention relates to electrochemical fuel cells, moreparticularly electrochemical fuel cells which employ hydrogen as a fueland receive an oxidant to convert the hydrogen to electricity and heat.This invention is even more particularly concerned with thehumidification requirements of such an electrochemical fuel cellemploying a proton exchange membrane.

BACKGROUND OF THE INVENTION

[0003] Generally, a fuel cell is a device which converts the energy of achemical reaction into electricity. It differs from a battery in thatthe fuel cell can generate power as long as the fuel and oxidant aresupplied.

[0004] A fuel cell produces an electromotive force by bringing the fueland oxidant into contact with two suitable electrodes and anelectrolyte. A fuel, such as hydrogen gas, for example, is introduced ata first electrode where it reacts electrochemically in the presence ofthe electrolyte and catalyst to produce electrons and cations in thefirst electrode. The electrons are circulated from the first electrodeto a second electrode through an electrical circuit connected betweenthe electrodes. Cations pass through the electrolyte to the secondelectrode. Simultaneously, an oxidant, typically air, oxygen enrichedair or oxygen,is introduced to the second electrode where the oxidantreacts electrochemically in presence of the electrolyte and catalyst,producing anions and consuming the electrons circulated through theelectrical circuit; the cations are consumed at the second electrode.The anions formed at the second electrode or cathode react with thecations to form a reaction product, such as water. The first electrodeor anode may alternatively be referred to as a fuel or oxidizingelectrode, and the second electrode may alternatively be referred to asan oxidant or reducing electrode. The half-cell reactions at the twoelectrodes are as follows:

First Electrode: H₂→2H⁺+2e ⁻

Second Electrode: 1/2O₂+2H⁺+2e ⁻→H₂O

[0005] The external electrical circuit withdraws electrical current andthus receives electrical power from the cell. The overall fuel cellreaction produces electrical energy which is the sum of the separatehalf-cell reactions written above. Water and heat are typicalby-products of the reaction.

[0006] In practice, fuel cells are not operated as single units. Rather,fuel cells are connected in series, stacked one on top of the other, orplaced side by side. A series of fuel cells, referred to as fuel cellstack, is normally enclosed in a housing. The fuel and oxidant aredirected through manifolds to the electrodes, while cooling is providedeither by the reactants or by a cooling medium. Also within the stackare current collectors, cell-to-cell seals and insulation, with requiredpiping and instrumentation provided externally to the fuel cell stack.The stack, housing, and associated hardware make up the fuel cellmodule.

[0007] Fuel cells may be classified by the type of electrolyte, which iseither liquid or solid. The present invention is primarily concernedwith fuel cells using a solid electrolyte, such as a proton exchangemembrane (PEM). The PEM has to be kept moist with water because theavailable membranes will not operate efficiently when dry. Consequently,the membrane requires constant humidification during the operation ofthe fuel cell, normally by adding water to the reactant gases, usuallyhydrogen and air.

[0008] The proton exchange membrane used in a solid polymer fuel cellacts as the electrolyte as well as a barrier for preventing the mixingof the reactant gases. An example of a suitable membrane is acopolymeric perfluorocarbon material containing basic units of afluorinated carbon chain and sulphonic acid groups. There may bevariations in the molecular configurations of this membrane. Excellentperformances are obtained using these membranes if the fuel cells areoperated under fully hydrated, essentially water-saturated conditions.As such, the membrane must be continuously humidified, but at the sametime the membrane must not be over humidified or flooded as thisdegrades performances. Furthermore, the temperature of the fuel cellstack must be kept above freezing in order to prevent freezing of thestack.

[0009] Cooling, humidification and pressurization requirements increasethe cost and complexity of the fuel cell, reducing its commercial appealas an alternative energy supply in many applications. Accordingly,advances in fuel cell research are enabling fuel cells to operatewithout reactant conditioning, and under air-breathing, atmosphericconditions while maintaining usable power output.

[0010] The current state-of-the-art in fuel cells, although increasinglyfocusing on simplified air-breathing, atmospheric designs, has notadequately addressed operations in sub-zero temperatures, which requiresfurther complexity in the design. For instance, heat exchangers andthermal insulation are required, as are additional control protocols forstartup, shut-down, and reactant humidifiers.

[0011] Where a solid polymer proton exchange membrane (PEM) is employed,this is generally disposed between two electrodes formed of porous,electrically conductive material. The electrodes are generallyimpregnated or coated with a hydrophobic polymer such aspolytetrafluoroethylene. A catalyst is provided at eachmembrane/electrode interface, to catalyze the desired electrochemicalreaction, with a finely divided catalyst typically being employed. Themembrane/electrode assembly is mounted between two electricallyconductive plates, each which has at least one fluid flow passage formedtherein. The fluid flow conductive fuel plates are typically formed ofgraphite. The flow passages direct the fuel and oxidant to therespective electrodes, namely the anode on the fuel side and the cathodeon the oxidant side. The electrodes are electrically connected in anelectric circuit, to provide a path for conducting electrons between theelectrodes. In a manner that is conventional, electrical switchingequipment and the like can be provided in the electric circuit as in anyconventional electric circuit. The fuel commonly used for such fuelcells is hydrogen, or hydrogen rich reformate from other fuels(“reformate” refers to a fuel derived by reforming a hydrocarbon fuelinto a gaseous fuel comprising hydrogen and other gases). The oxidant onthe cathode side can be provided from a variety of sources. For someapplications, it is desirable to provide pure oxygen, in order to make amore compact fuel cell, reduce the size of flow passages, etc. However,it is common to provide air as the oxidant, as this is readily availableand does not require any separate or bottled gas supply. Moreover, wherespace limitations are not an issue, e.g. stationary applications and thelike, it is convenient to provide air at atmospheric pressure. In suchcases, it is common to simply provide channels through the stack of fuelcells to allow for flow of air as the oxidant, thereby greatlysimplifying the overall structure of the fuel cell assembly. Rather thanhaving to provide a separate circuit for oxidant, the fuel cell stackcan be arranged simply to provide a vent, and possibly some fan or thelike to enhance air flow.

[0012] Catalytic burners are also known and operate on a principlesimilar to fuel cells, but at an accelerated kinetic rate and increasedtemperature. A fuel, for example hydrogen, is oxidized through directcontact with oxygen or air at a rate induced by the presence of acatalytic bed, for example, ceramic beads containing small amounts ofplatinum on the surface.

[0013] The by-product of the chemical reaction is similar to that of afuel cell, but without any generation of electricity:

O₂+2H₂→2H₂O+HEAT

[0014] The higher consumption rate of the reactants and concomitant heatrelease reflects the fact that the reaction occurs through directcontact rather than through a proton/electron transaction. Catalyticburning is flameless, and occurs at a temperature between that of a fuelcell's “cold combustion” and that of an open-flame combustion. Flow ratecan be pulsed or modulated to achieve varying heat and moistureprofiles. Hydrogen catalytic burning requires no pilot flame or spark tobe initiated.

[0015] An example of a proposal for a catalytic burner is found in anarticle entitled “Catalytic Combustion of Hydrogen in a DiffusiveBurner” by K. Stephen and B. Dahm at pages 1483-1492 of CatalyticCombustion of Hydrogen in a Diffusive Burner.

SUMMARY OF THE INVENTION

[0016] In the prior art, humidification through membrane stack providedhumidification only, without imparting significant temperature shift. Inthe present invention, both humidification and temperature are impartedto the stream prior to admission to the stack.

[0017] In accordance with a first aspect of the present invention, thereis provided a tubular reactor, for catalyzing reaction of hydrogen and agaseous oxidant, the tubular reactor comprising:

[0018] an elongated housing, a catalyst formed from a material adaptedto promote catalytic combustion of the fuel and the oxidant, beingformed into an elongated body substantially filling the elongate housingand being porous, a first inlet for a gaseous fuel and a second inletfor a gaseous oxidant, both first and second inlets being provided atone end of the elongated housing;

[0019] and an outlet at the other end of the housing, whereby, in use,the catalyst promotes combustion between the fuel and the oxidant togenerate heat and moisture, whereby a heated and humidified gas flowexits through the outlet.

[0020] Preferably, the housing and the body of the catalyst are bothgenerally cylindrical and have length substantially longer than thediameter than the tubular reactor.

[0021] In accordance with a second aspect of the present invention,there is provided a fuel cell system comprising at least one fuel cell,each fuel cell comprising:

[0022] an inlet for a fuel;

[0023] an anode having a catalyst associated therewith for producingcations from the fuel;

[0024] a fuel manifold, connected between the inlet and the anode, fordistributing fuel to the anode;

[0025] an oxidant inlet means for supplying oxidant;

[0026] a cathode having a catalyst associated therewith and connected tothe oxidant inlet means, for producing anions from the oxidant, saidanions reacting with said cations to form water on said cathode;

[0027] an ion exchange membrane deposed between said anode and saidcathode, said membrane facilitating migration of cations from said anodeto said cathode, while isolating the fuel and the oxidant from oneanother;

[0028] a catalytic reactor having a first inlet for fuel and a secondinlet for an oxidant, and an outlet for heated and humidified gas, thecatalytic reactor being mounted to supply the heated and humidified gasto the fuel cell.

[0029] Preferably, the fuel cell system comprises a plurality of fuelcells, forming a fuel cell stack.

[0030] The stack can comprise an air-breathing stack, including aplurality of channels extending through the fuel cell stack forpermitting free flow of ambient air as the oxidant through the fuel cellstack, there being at least one channel for each fuel cell, wherein thecatalytic reactor is mounted below the fuel cell stack. The catalyticconverter is configured to receive air as an oxidant through the secondinlet thereof in excess of the stoichiometric quantity of air requiredfor combustion of fuel within the catalytic reactor, whereby heated andhumidified air is discharged from the outlet of the catalytic reactor.The outlet of the catalytic reactor is mounted below the channels of thefuel cell stack, whereby heated and moistened air flows upwardly throughthe channels of the fuel cell stack from the catalytic reactor.

[0031] The catalytic reactor can be either generally tubular or it canbe disk-shaped, configured for flow of fuel and oxidant generally alongthe central axis of the reactor.

[0032] A further aspect of the present invention provides a method ofoperating a fuel cell system comprising a plurality of fuel cells, eachfuel cell comprising an inlet for fuel, an anode having a catalystassociated therewith for producing cations from fuel, a fuel manifoldconnected between the inlet and the anode for distributing fuel to theanode, an oxidant inlet means for supplying oxidant, a cathode having acatalyst associated therewith and connected to the oxidant inlet meansfor producing anions from the oxidant, said anions reacting with saidcations to form water on said cathode and an ion exchange membranedisposed between the anode and the cathode, for facilitating migrationof cations from the anode to the cathode, while isolating the fuel andoxidant from one another, the method comprising

[0033] (a) supplying oxidant and fuel to the fuel cell for reaction togenerate electrical power and heat;

[0034] (b) supplying fuel to the catalytic reactor and oxidant to thecatalytic reactor, in an amount greater than the stoichiometric amountrequired for the combustion of the fuel, to ensure complete combustionof the fuel, thereby generating a flow of heated and humidified oxidant;

[0035] (c) supplying the heated and humidified oxidant to the fuel cell,for reaction with the fuel to generate electricity and heat.

[0036] For initial start-up below a preset temperature, the method cancomprise initially supplying fuel and oxidant only to the catalyticreactor to generate a flow of heated and moistened oxidant, and passingthe heated and moistened oxidant through the fuel cell to preheat thefuel cell, and commencing supply of fuel to the fuel cell, once the fuelcell reaches a desired temperature. Then, after start-up and after thefuel cell has reached the desired temperature, a sufficient quantity ofthe oxidant and the fuel are supplied to the reactor, to maintain theoxidant supplied to the fuel cell system at a desired humidity level.

[0037] Yet another aspect of the present invention provides a method ofoperating a fuel cell system comprising a plurality of fuel cells, eachfuel cell comprising an inlet for fuel, an anode having a catalystassociated therewith for producing cations from fuel, a fuel manifoldconnected between the inlet and the anode for distributing fuel to theanode, an oxidant inlet means for supplying oxidant, a cathode having acatalyst associated therewith and connected to the oxidant inlet means,for producing anions from the oxidant, said anions reacting with saidcations to form water on said cathode and an ion exchange membranedisposed between the anode and the cathode, for facilitating migrationof cations from the anode to the cathode, while isolating the fuel andthe oxidant from one another, the method comprising:

[0038] (a) supplying oxidant and fuel to the fuel cells for reaction togenerate electrical power and heat;

[0039] (b) supplying fuel to the catalytic reactor and oxidant to thecatalytic reactor, in an amount less than the stoichiometric amountrequired for combustion of fuel, to ensure complete consumption of theoxidant, thereby generating a flow of heated and humidified fuel;

[0040] (c) supplying the heated and humidified fuel to the fuel cell,for reaction with oxidant known to generate electricity and heat.

[0041] This aspect of the method can include:

[0042] (a) providing a second catalytic reactor;

[0043] (b) supplying the second reactor with fuel and oxidant in anamount greater than the stoichiometric amount required for combustion offuel, thereby generating a flow of heated and humidified oxidants;supplying the heated and humidified oxidant to the oxidant inlet meansof the fuel cell, for reaction with a heated and humidified fuel togenerate electricity and heat. In the prior art, humidification throughmembrane stacks provided humidification only, without imparting asignificant temperature shift. In this invention, both humidity andtemperature are imparted to the stream prior to admission to the stack.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0044] For a better understanding of the present invention and to showmore clearly how it may be carried into effect, reference will now bemade, by way of example, to the accompanying drawings which show apreferred embodiment of the present invention and in which:

[0045]FIG. 1 is a schematic view of the first embodiment of a fuel cellsystem in accordance with the present invention;

[0046]FIG. 2 is a schematic view of a second embodiment of a fuel cellsystem in accordance with the present invention;

[0047]FIGS. 3a and 3 b are, respectively, perspective and side views ofa tubular reactor in accordance with the present invention;

[0048]FIG. 4 is a plan view of part of the fuel cell stack of FIGS. 1and 2; and

[0049]FIG. 5 is a more detailed view of the second embodiment of thefuel cell system shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0050] Referring first to FIG. 1, the first embodiment of the apparatusis indicated generally by the reference 10 and includes an enclosure 12,in the apparatus. In the drawings, this is identified as a HyTef-FC15Enclosure.

[0051] Within the enclosure 12, there is a fuel stack 14 comprising, inknown manner, a plurality of PEM fuel cells, and described in greaterdetail in relation to FIG. 4. For the stack 14, a main fuel supply line16 is provided for hydrogen. The fuel line 16 extends into the enclosure12 and continues as a main supply line 18 including a respective flowcontrol valve 24 and a solenoid-operated valve 25. As shown, a stackpurge outlet at 26 enables excess hydrogen to be purged from the fuelcell stack 14. A respective solenoid controlled valve 27, controlspurging of the hydrogen gas. Again, as is known, this preventsaccumulation of contaminants and impurities in the hydrogen fuel, withinthe fuel cell stack 14. The purged hydrogen through the purge outlet 26can be recycled for consumption.

[0052] The fuel cell stack 14 in FIG. 1 is a closed stack. Hydrogen fuelflows through the anode side of each individual fuel cell in knownmanner. Correspondingly, an air inlet 34 is provided, connected to anair line 38. A pump 36 for the air is provided, and an air exhaustindicated at 43.

[0053] In accordance with the present invention, the fuel or hydrogensupply line 16 is connected to a catalytic reactor 50, which includes acatalytic reactor bed 57 (FIG. 3), comprising, for example reticulatedaluminum; this material is chosen for its thermal conductivity, cost andease of use. A separate air inlet 41 is provided, connected via a pump40 and an air supply line 42 to the catalytic reactor 50. Non-returnvalves 58 prevent back flow of air and fuel, and a flash arrestor 59 isprovided for the fuel cell.

[0054] The catalytic reactor 50 is generally tubular, has respectiveinlets 52 and 54 for hydrogen and air, and a tubular outlet 56. A flowof heated, humidified fuel exits from the tubular outlet 56, and willthen flow to the fuel inlet of the fuel cell stack.

[0055] Reference will now be made to FIGS. 2 and 5, which shows a thirdembodiment of the present invention. This embodiment of the inventionagain can have an enclosure, as indicated at 60, and again includes afuel cell stack, here indicated at 62. The stack 62 here is a closedstack, and is provided with an air pump or blower 64 connected by a mainsupply line 66 to an inlet of the fuel cell stack 62, and excess airexhausts from the fuel cell stack 62 as indicated at 68.

[0056] On the hydrogen side, a hydrogen supply line 70 can include apressure gauge and a flow meter (not shown), and comprises a mainhydrogen supply line 72 to the fuel cell stack 62 and a secondary supplyline 74 to the catalytic burner or reactor. A solenoid valve 73 isprovided in the main supply line 72, and a solenoid valve 75, a flasharrestor 76 and a non-return valve 77 are provided in the secondary line74. A fuel purge valve 78 with a controlling solenoid valve 79 areprovided as for the first embodiment.

[0057] The tubular, catalytic reactor 50 is again provided and thehydrogen inlet 52 is again provided at the side of the reactor.

[0058] An air supply line for the catalytic reactor 50 is indicated at80 and includes a pump or meter 82, and a respective non-return valve84. The air supply line 80 is connected to a respective inlet 54.Optionally, a pressure gauge and a flow meter can be provided.

[0059] The outlet 56 of the tubular reactor 50 is connected by a line85, to two branch lines 86 and 87, which are connected by respectivesolenoid valves 88 and 89 to the supply line 72 and to the air supplyline 66. Although not shown, the stack 62 can optionally include arecirculation pump. Excess hydrogen can, in a known manner, be purgedthrough the outlet 68 or purge line 78, to prevent build-up ofcontaminants.

[0060] The tubular reactor 50 can be run to provide either a humidifiedand heated flow of air or a humidified and heated flow of hydrogen.These two modes of operation are detailed below.

[0061] To generate a flow of heated and humidified air, excess air isdelivered by the pump 82, relative to the hydrogen flow through the line74. In the tubular reactor 50, the oxygen reacts with the hydrogen togenerate heat and moisture. This results in a heated and moistened airflow exiting through the outlet 56. Then, the valve 88 is maintainedclosed and the valve 89 is opened, so that the heated and moistened airflow passes through to the main air supply line 66, to be entrained intothe air flow passing to the fuel cell stack 62.

[0062] Correspondingly, to generate a heated hydrogen flow, the valve 88is opened and the valve 89 closed. Then, excess hydrogen is suppliedthrough the line 74, as compared to air supplied through the main fuelline 82. The flow is dead ended and is only exhausted during purgingwhen the exhaust solenoid is open. However, the flow can be controlledusing control valves when not operated in dead-ended mode. In thetubular reactor 50, the oxygen in the air reacts with some of thehydrogen to generate heat and moisture. The flow of hydrogen, withresidual nitrogen, together with heat and moisture, then exits from theoutlet 56. This flow of heated and humidified nitrogen and hydrogen gaspasses through valve 88 into the main fuel line 72.

[0063] It will be appreciated that where heated and humidified hydrogenis supplied to the fuel line 72, and as air is used as the oxidant, thisdoes result in nitrogen being injected into the fuel gas supply. Forthis reason, the purge line 78 will need to be used, to prevent thebuild-up of nitrogen within the fuel cell stack 62. Alternatively, aflowing system can be used at all times.

[0064] It is important that, in the tubular, catalytic reactor 50,complete reaction takes place. In other words, it is essential that, inthe two modes of operation, residual hydrogen is not delivered to themain air line 66, nor residual oxygen delivered to the hydrogen supplyline 72. This could result in potentially flammable gas mixtures ofhydrogen and oxygen being delivered to the fuel cell stack 62, which isdangerous. To ensure complete reaction, proper topology and morphologyof the reactor must be designed, essentially to ensure adequateresidence time over the full range of flow rates.

[0065] It will also be understood that it is possible to heat andhumidify both of the fuel and air supply lines. Because of the differentrequirements of the two supply lines, this would require the provisionof two separate tubular reactors, each of which would be configured tooperate in one of the two modes outlined above.

[0066] Turning to FIG. 3, this shows, in detail, the tubular reactor 50.It is to be appreciated that this is an early version of the tubularreactor 50, and in particular, the housing of the tubular reactor 50 ismade from conventional, off-the-shelf components. It is anticipated thatthe overall configuration of the tubular reactor 50 can be enhanced togive a design which both has better performance characteristics, and ismore economical to manufacture.

[0067] The tubular reactor 50 comprises a tubular reactor housing 51. Atthe lower end thereof, a T-connector 100 is provided. The T-connector100 has three coupling flanges 102, one of which is connected to thetubular housing 51, and the two others of which provide connections forthe hydrogen supply lines. At the top end, the tubular reactor 50includes a connector 104, again provided with connection flanges 106,one of which is connected to the tubular housing 51 and the others ofwhich provide connections to supply lines. While a housing 51 ofcircular cross-section is shown, it will be understood that any suitablecross-section, for example a square cross-section, could be used.

[0068] Reference will now be made to FIG. 4. This shows a plan view of,for example five pairs of flow field plates making up five individualfuel cell elements in the fuel cell stack 62. Thus, there are oxidantflow field plates indicated at 110. Fuel flow field plates are indicatedat 112. Between each pair of oxidant and fuel flow field plates 110,112, there is located a respective membrane electrode assembly (MEA) andgas diffusion media 114. Between the oxidant flow field plates 110 andthe MEA 114, there are defined oxidant channels 116, and fuel flowhydrogen channels 118 are defined between the fuel flow field plates 112and the MEA 114. Cooling channels 120 are provided in the back of theoxidant flow field plates 110, against the fuel flow field plates 112.These cooling channels 120 are, like the oxidant channels 116, simplychannels extending vertically (not necessarily vertical) through thestack 62, to provide free flow of ambient air through the channels.Thus, a stack with this configuration, is intended as an air-breathingstack, as mentioned above, and can be incorporated into the embodimentsof the earlier figures. In known manner, other constructional details ofthe stack, e.g. elements holding the various flow field plates together,are not shown, but these can be conventional.

1. A tubular reactor, for catalyzing reaction of hydrogen and a gaseousoxidant, the tubular reactor comprising: an elongated housing, a firstinlet for a gaseous fuel and a second inlet for a gaseous oxidant, bothfirst and second inlets being provided at one end of the elongatedhousing and an outlet at the other end of the housing; and a catalystformed from a material adapted to promote catalytic combustion of thefuel and the oxidant, being formed into an elongated body substantiallyfilling the elongated housing and being porous, whereby, in use, thecatalyst promotes combustion between the fuel and the oxidant togenerate heat and moisture, whereby a heated and humidified gas flowexits through the outlet.
 2. A tubular reactor as claimed in claim 1,wherein the housing and the body of the catalyst are both generallycylindrical and have a length substantially longer than the diameterthereof.
 3. A tubular reactor as claimed in claim 1 or 2, whichincludes, for the first and second inlets, fittings for connection tosupply lines for fuel and the oxidant, and for the outlet, a fitting forconnection to a conduit for receiving the heated, humidified gas flow.4. A tubular reactor as claimed in claim 3, wherein the fittings for thefirst and second inlets, comprise a T-connector including 3 couplingflanges, one being connected to the tubular housing and the other twoflanges providing the first and second inlets, and the fitting for theoutlet comprises a connector with a pair of flanges, one flange beingconnected to the tubular housing and the other flange of the connectorforming the outlet.